FIG. 1 illustrates a conventional string of LEDs (LED1-LEDN) driven by a supply voltage source 12 and a current source. In the example of FIG. 1, the current source is a MOSFET 14 whose conductivity is controlled using a current detector 16 (e.g., a low value resistor), a controller 18, and an Iset signal. The voltage drop across the detector 16 is compared to a reference, provided by the Iset signal. The controller 18 controls the MOSFET 14 to cause the voltage drop to correspond to the Iset signal. Many other types of current controllers can be used.
The brightness of the LEDs is controlled by controlling the current through the LEDs. The voltage supplied by the voltage source 12 must be at least as great as the total voltage drop across all the LEDs plus the voltage needed for operation of the current source. The voltage drop of conventional LEDs is between 2-4 volts. Depending on the type of LED, the currents can range from 20 mA-100 mA, for low power LEDs, to 300 mA-1 A for high power LEDs.
LEDs are frequently connected in series and parallel, depending on the available power supply voltage, the required brightness, the colors to be controlled, and other factors. One increasingly popular use of LEDs is in a light fixture, driven by household current, where many LEDs are connected in series due to the high voltage. Connecting multiple LEDs in series is also common for large backlights of LCDs where high brightness is required, and where LEDs of the same color (e.g., red, green, or blue) are connected in series so they can be controlled using a single current source for each individual color. LEDs of different colors have different electrical characteristics, such as voltage drops, since they are formed of different materials.
Since LEDs of different colors and from different manufactures have different electrical characteristics, it is difficult to design an efficient LED drive system that can be used with any type of LED. Inefficiency increases when excess power supply voltage is used since the excess voltage is dropped across the current source MOSFET. The prior art systems require excess voltage when driving a serial string of LEDs since, if the supply voltage is even barely insufficient to drive the entire string of LEDs, all the LEDs are off.
In cases where the supply voltage is not regulated, such as a battery or a rectified AC signal, all the LEDs in the string will be turned off once the instantaneous supply voltage level drops below a threshold level.
It would be desirable to have an efficient LED driver for driving many LEDs, of any type, where only those LEDs that can be driven by the power supply are energized. It is also desirable to have an LED driver that can use a rectified AC voltage where all the LEDs do not turn off together once the instantaneous AC voltage drops below a threshold.